Middle Devensian deposits beneath the 'Upper Floodplain' terrace ofthe River Thames at Kempton Park, Sunbury, England
P. L. Gibbard, G. R. Coope, A. R. Hall, R. C. Preece and J. E. RobinsonGIBBARD, P. L., G. R. COOPE, A. R. HALL, R. C. PREECE & J. E. ROBINSON. 1981.Middle Devensian deposits beneath the 'Upper Floodplain' terrace of the River Thames atKempton Park, Sunbury, England. Proc. Geol. Ass., 93 (3), 275-289. Gravel pit sections in the'Upper Floodplain' terrace exposed current-bedded gravel and sand containing a grey clayey siltlens. This silt has yielded fossil macroscopic plant remains, molluscs, ostracods and insects.Sedimentological and palaeontological results indicate that the silts were deposited in a slowlyflowing river channel. The gravel and sand accumulated under cool, seasonally contrasting climaticconditions and represent braided stream deposits. A new lithostratigraphical unit, the KemptonPark Gravel, is proposed and its relation to the local terrace stratigraphy of the Thames isdiscussed. A radio-carbon date of 35,230 185 BP indicates that the Kempton Park Gravel unitaggraded during the Middle Devensian substage.
P. L. Gibbard, Subdepartment of Quaternary Research. Botany School. Downing Street,Cambridge CB23EA.G. R. Coope, Department of Geological Sciences. University of Birmingham. P.O. Box 363,Birmingham B152TT.A. R. Hall. Environmental Archaeology Unit, University of York. Heslington, York YO] 5DD.R. C. Preece. Department of Geology. Imperial College. Prince Consort Road, London SW72BP.J. E. Robinson, Department of Geology. University College, Gower Street, London WC] E 6BT.
1. INTRODUCTIONDuring summer 1978, at Kempton Park, Sunbury,Surrey, a bed of grey clayey silt containing fossilplant and animal remains was discovered in a gravel pit(TQ 118703). The pit was excavated in the RiverThames 'Upper Floodplain' terrace deposits (Dewey &Bromehead, 1921) which have been extensively workedin the district. The pit is located immediately east of theKempton Park Race Course and south of the Hampton-Shepperton railway line (Fig. 1). The ground surface atthe site is at 12 mOD.
The local geology and internal terrace stratigraphy ofthe area have been very little studied. Dewey & Bro-ne-head (1915, 1921) mapped the surfaces and deposits ofterraces of the district, including the 'Upper Floodplain'terrace, the existence of which was supported by Zeuner(1959). More recently, Coope & Angus (1975) describedMiddle Devensian organic deposits containing faunaland floral remains downstream at Isleworth. These de-posits were previously reported from nearby Twicken-ham by Leeson & Laffan (1894). The dating of the'Upper Floodplain' terrace aggradation is complicatedby the occurrence of Ipswichian interglacial deposits atBrentford (Zeuner 1959) and Trafalgar Square (Franks,Sutcliffe, Kerney & Coope, 1958; Franks 1960) beneaththe terrace. The aim ofthis paper is to present the resultsof investigations of the Kempton Park deposits andtheir bearing on the geological history of the depositsbeneath the 'Upper Floodplain' terrace of the RiverThames.
2. SITE DESCRIPTIONThe deposits exposed at the Kempton Park site in sum-mer 1978 are summarised in Fig. 2. The sequence con-sists of up to 4 m of current-bedded gravel and sand thatlies on an irregularly channelled surface of London Clay.The sediments comprise predominantly horizontallybedded clast-supported gravels that in places containnarrow, laterally impersistent lenses of current-beddedsand up to 20 em in thickness. They are almost exclu-sively traction load sediments laid down in broad, shal-low, frequently cross-cutting channel and bar-like struc-tures under highly variable flow conditions. Thesedimentary structures and facies relationships suggestthat the gravel and sand was probably deposited by abraided river (Williams & Rust, 1969; Rust, 1972) thatflowed towards the ENE, as shown by palaeocurrentmeasurements. A series of gravel samples for pebblecounting in the 32-8 mm size range gave a uniformcomposition of 93 per cent flint, 5 per cent vein quartz, 1per cent quartzite and 1 per cent Lower Greensand chert(using 300-600 pebbles in each of four samples, cf. Gib-bard 1974). Over most ofthe western face of the quarry,22 m above the base (at point A), a grey clayey silt bedcontaining macroscopic plant remains, mollusc shells,ostracod valves and insects is intercalated in the gravel.The bed rests on the gravel beneath with a sharplydefined base and is in turn succeeded by further graveland sand strongly channelled into the clayey silt. Thiserosional base of the overlying gravel results in theclayey silt bed being of variable thickness; in places it is
276 P. L. GIBBARD, G. R. COOPE, A. R. HALL, R. C. PREECE AND J. E. ROBINSON
I II I Islope between terracesT Taplow Terrace
UFPUpper Floodplain TerraceLFP Lower Floodplain Terrace
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tFig. 1. Location map showing the position of the site, its relation 10 local terraces and the line of the section shown in Fig. 4.
B C A
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Grey siltGravel and
Kempton Park 1978
Fig. 2. Section at Kempton Park pit in summer 1978. The relative positions of the four profiles is shown above .
MIDDLE DEVENSIAN DEPOSITS OF RIVER THAMES 277
cut out entirely, so that the overlying gravel comes torest directly on that beneath. The two gravel sub-unitsare identical in structure, palaeocurrent direction andcomposition. There is therefore no reason to supposefrom the sedimentary evidence that the clayey silt bedrepresents a major hiatus in deposition, i.e. the entiresequence represents a single aggradational event.
The section is capped by up to 26 m of floodplainalluvial deposits that appear to fill a channel, since theythin towards the west. Dark grey silty clay 20 em thickoccurs at the base and rests directly on the gravelbeneath. This is overlain by 11 m of brown clayey silt,which is in turn succeeded by 13 m of light brown clayeysilt. Beneath the ground surface the clayey silt is weath-ered and is mottled light grey and brown in colour.
3. GREY CLAYEY SILT BED: COMPOSITIONAND STRUCTURE
Detailed investigation of the fossiliferous grey clayey siltbed in the gravels shows that it represents a channel-fillthat accumulated predominantly under slow flowingwater conditions. The silt is laterally variable in thick-ness reaching a maximum of 70 em at profile point A,16 em at profile point B, 40 em at profile point C and32 em at profile point D. The composition of the sedi-ments analysed from this unit is shown below using thenotation system of Troels-Smith (1955). Because of thelateral variability and irregular distribution of fossil re-mains four localities were sampled, the relative positionsof which are shown in Fig. 2.
The sequences are as follows:
Orange gravel and sand
Depth 0-23 Light grey mottled black clayey silt withisolated pebbles and plant fragments(AsH, Ag2, DI~
23-34 Grey current bedded sand
34-66 Light grey clayey silt with narrow sandbands, with a few shell fragments andplant remains (As1, Ag2!, DI~)
66-70 Brown oxidised clayey silt (As1, Ag3)Orange gravel and sand
0-16 Light grey clayey silt with a number oflight grey silty sand band, plant fragmentsat 8-9 em (As1, Ag2, Dll)Gravelly sand
Orange sand and gravel
0-15 Light grey clayey silt (As1, Ag3, Dl+)
15-24 Orange gravel and sand much disturbedby loading to form 'ball-and-pillow' struc-tures
24-40 Light grey clayey silt with narrow sandbands and bands of plant and shell frag-ments 1-2 mm thick (As1, Ag2!, DI!)Orange gravel and sand
Orange gravel and sand
0-32 Light grey clayey silt with narrow sandbands disturbed near base by possiblebioturbation. Plant detritus present inisolated band near top of unit 0-5 em.(As1, Ag2, Dll)
Orange gravel and sand
From these sections it emerges that the grey clayeysilt fines upwards becoming slightly more clayey andricher in plant remains in the upper parts. A series ofsand or sandy silt bands ranging from less than 05 toover 3 em in thickness subdivide the bed. The bandsconsist of a basal coarse member resting on an erodedbase. This member fines upwards into a grey clayey siltwhich contains an irregular distribution of plant andanimal remains. At least three prominent and sevenminor bands were identified at the northern end of thesection. In places, the sand bands become thicker andcoarser, passing laterally into pebbly sand or fine gravel.One such band reached a maximum thickness of 10 ern(Fig. 2). Where this band consisted of gravel, load cast-ing had occurred resulting from the silt beneath havingbeen injected into the gravel above forming small 'ball-and-pillow' type structures. Load casting of this typemust have occurred during emplacement of the mainoverlying gravel unit and indicates that the sedimentswere not frozen at the time. The presence of high energysediments in the otherwise low energy sequence indi-cates that periodic flooding of the channel took placethroughout the time represented. In at least one casewater flowwas rapid enough to deposit gravel and coarsesand. Isolated pebbles occurred in the grey clayey silthorizons either singularly or in groups of two or three. Itis difficult to explain the